Hi sorry it’s not building regs to have a fan if you have a opening window in wc although any competent electrician would recommend installing a fan in a small downstairs wc, hope that helps!

If an existing w/c doesn’t have a fan then it’s not a regulation to have one installed. If it’s a new build or extension then yes you require a fan to be installed. Air Extractor fans in the w/c Regulations require a fan capable of a minimum extract capacity of 6 litres per second 21m3/hr (12CFM) when installed. If a room does not contain an openable window the fan should be fitted with a 15 minute over-run timer. Hope this helps

You initially say there is no window then you say the developer says there’s a window? Who is correct?

As you have no window, then yes there should be ventilation. I would put this to the builder or the electrician (Depending who hired who) and strongly suggest that they 'finish the job' as per building regulations. How ever it seems that the builders are washing their hands of job. I assume you have paid them?
If you have no joy with the builder, find out who they are accredited with and contact them. They will make the builder finish the work or get someone else to do it, at their cost.
Same with the electrician and if they did any work in the room then you should at least have a minor works certificate. Technically, the builder should have put it on the plans, and the electrician should have fitted it. Both are at fault.
If they have no governing body then you will have to go through the council inspectors.
All this is how ever a lot of hassle. You may just be better off getting a recommended professional in to install the fan.

Hope this helps and good luck.

Do you have a window in it or not?

One aim of the building regs is to not make the building any worse than found. I would say that converting a room into a WC would go against that. Definitely required in a new build, but also in this instance. A window in the living area is not acceptable and irrelevant.